Recently, along with the advancement in communication using electromagnetic waves in the microwave frequency region, for example, in automobile telephones, in portable telephones, or in satellite broadcasts, the terminal apparatus is required to be smaller. In order to attain this goal, each component comprising the terminal apparatus must be miniaturized. In those apparatus, a dielectric ceramic is inserted in filters or in resonant devices. When using the same resonance mode, the size of the dielectric resonator is inversely proportional to the square root of the dielectric constant (.epsilon..sub.r) attributed to the dielectric material. Therefore, a material having a high dielectric constant is needed to manufacture a compact-sized dielectric resonator. In addition, the dielectric material for the dielectric resonator must have low loss in the microwave region. In other words, the dielectric material must have a high Q value and a low temperature coefficient (.tau..sub.f) of a resonance frequency. Q value means reciprocal of dielectric loss tan .delta..
As materials with a high dielectric constant, (Pb,Ca) ZrO.sub.3 are disclosed in Published Examined Japanese Patent Application No. (Tokkai Hei) 4-65021. These materials have high dielectric constants over 100, high Q values of about 800 in 2-4 GHz, and lower temperature coefficients of resonance frequencies.
On the other hand, conductors and dielectric ceramics are laminated to miniaturize a resonant device. The conductor should have a higher conductivity when used in a high frequency (e.g. microwave) region. Therefore Cu, Au, Ag, or alloys of such metals should be used. These metals must be sintered carefully in a condition where the metal of the conductor does not melt or oxize, because the dielectric ceramic need to be fired carefully along with the metal conductors to be laminated. The material must be fired at lower temperatures (for Cu, at 1083.degree. C. or less; Au, at 1063.degree. C. or less; Ag, at 961.degree. C. or less), and when using Cu for an electrode, the partial pressure of oxygen should be lower. Bi.sub.2 O.sub.3 --CaO--Nb.sub.2 O.sub.5 has been disclosed in U.S. Pat. No. 5,273,944, as a microwave dielectric ceramic.
Though the above-mentioned ceramic made of Bi.sub.2 O.sub.3 --CaO--Nb.sub.2 O.sub.5 can be sintered at about 1000.degree. C., the first component (Bi.sub.2 O.sub.3) evaporates when fired. Thus the dielectric properties, with regard to the firing temperature, are unstable.